1. Field of the Invention
This invention relates to hydrophilic plastic materials useful as soft contact lenses, and more particularly relates to treatment of said hydrophilic polymer lenses to make them more resistant to clouding and discoloration in use.
2. The Prior Art
It has long been known to fabricate contact lenses from rigid materials such as glass and clear plastics, e.g., essentially hydrophobic acrylic-type polymers as polymethyl methacrylate and the like. Although more safely handled and used than glass, such hydrophobic plastic materials have been only moderately successful as contact lenses, being too hard and uncomfortable to the wearer. More recently, contact lens compositions have been developed from hydrophilic-type polymers which are softer and generally more easily accommodated by the eye. Thus, the use of hydrophilic polymer lens compositions is becoming of increasing importance in opthalmological practice.
Hydrophilic polymers useful as soft contact lenses typically are lightly cross-linked, essentially water-insoluble copolymers derived from one or more monomers containing hydroxy groups which impart to the polymers their affinity for water. These polymers may further be defined as coherent, three-dimensional polymer structures or networks which have the ability to absorb or imbibe water, even in large quantities, e.g. up to 90 weight percent, without dissolution. When containing water in any amount whatsoever, a hydrophilic polymer will expand correspondingly and correctly may be designated as a hydrophilic polymer "gel." More specifically, a hydrophilic polymer containing imbibed water is known generally in the art as a "hydrogel." At the present time, the specific class of polymer hydrogels which have gained particular commercial acceptance as soft contact lenses are those derived from acrylic esters. U.S. Pat. Nos. 2,976,576 and 3,220,960 issued to O. Wichterle and D. Lim on Mar. 28, 1961 and on Nov. 30, 1965, respectively, are early patents which describe the use of methanol-insoluble hydrophilic acrylic ester polymeric materials for the manufacture of soft contact lenses. Many subsequent patents as well as other technical articles ar e directed to the preparation of numerous other hydrophilic acrylic ester-type polymers which differ primarily in the type and/or percentage of comonomers contained therein.
In the main, acrylic ester hydrophilic polymers are all derived by copolymerizing a major amount of a water-soluble monoester of acrylic or methacrylic acid in which the ester moiety contains at least one hydrophilic group and a minor amount of a bifunctional diester of acrylic or methacrylic acid which cross links the hydrophilic group-containing monomer as it polymerizes. The degree and type of cross-linking in the resulting polymer governs, to a large extent, its maximum water content, when fully hydrated.
Although presently used contact lenses fabricated from polymer hydrogels are much softer than the prior hard contact lenses and can be accommodated by the wearer with relatively little discomfort, they nevertheless have disadvantageous properties and likewise have not been completely satisfactory. Hydrogel lenses favor the growth of pathological bacteria and fungi on their surfaces. If the lenses are not regularly cleaned and sterilized, or if they are stoed in contaminated solutions, the pathogens can be easily sorbed by the lens material due to its flexible, hydrophilic polymer structure. Corneal-damaging chemical residues from improper cleaning and/or sterilization techniques can likewise be introduced into the lens material in this way. Also, because of its aforesaid flexible, hydrophilic polymer structure, proteins and other normal substances in the eye environment can be easily diffused through the lens with use. Accumulation of such substances in the lens causes its discoloration and clouding with repeated cleaning and sterilization techniques practiced by the wearer. Further, the lenses can lose sufficient amounts of water during use to deleteriously affect their dimensional stability and optical acuity.
It is object of this invention, therefore, to provide a soft contact lens which is resistant to penetration by pathological organisms and chemicals damaging to the eye, while simultaneously transmitting other substances beneficial thereto.
It is another object of this invention to provide a soft contact lens wherein the diffusion of proteins and other migratory eye substances is significantly inhibited so as to lengthen its life and optical effectiveness.
It is still a further object of this invention to provide a soft contact lens which will retain a sufficient quantity of water during use to maintain its dimensional stability and optical acuity.
These and other objects of this invention will become apparent to those skilled in the art from the description of the invention which follows.